Hi, They are both three speed. The BW35 is just about on its limit behind the V8. The BW65/66 is more heavily built and so has a longer service life. They do drive slightly different but not enough to notice in use. Gearboxes like the ZF are a step change to use.
The first car the BW35 was fitted to in the UK was the Daimler 2.5 litre V8 saloonas far as I remember. The first car I knew of and worked on fitted with the the BW65 was the Triumph Dolomite new in 1971/2. Jags may have had them around the same time, as well as BMW possibly. P6B got the BW65 in October 1973, (for the 1974 model year IIRC) so an "M" registered car in the UK could have had a 35 or a 65. The 2000/2200 never got the BW65.
There's plenty of info on the BW65 on this forum, you just have to ask.
that comment takes me back, in the 80's i knew a bloke who had a P5b, the engine was rebuilt with a hotter cam, ported heads and holley 4 barrel so something like 190-200bhp, it used to east gearboxes with great regularity, even gearboxes with lifetime warranty would only last about 6 months!
I always find the term "lifetime warranty" or "sealed for life" rather vague. People tend to think in human terms, like when we say "once in a lifetime" but in reality it is related with the life of the object that the warranty is applied to, which it could well be rather more short than one expects!
In Australia, the BW35 appeared behind numerous 2 and 3 litre engines, plus larger 4.1 litre Ford and 4.3 litre Valiant engines. My Rover's BW35 transmission was rebuilt to M51 specifications during 2009, and has run faultlessly (typing very quietly so it doesn't hear me haha) covering over 173,000 miles (279,000km) to date behind the 4.6, exceeding the distance covered by all previous rebuilds by almost 3 times, and they were all behind the 3.5 litre engine.
With the Valiant, I had a look and found this advertisement for a rebuilt BW35 designed to handle 400 HP! It is certainly not cheap at close to $4000 (say 2000 Pounds), but it sure must be different to the versions behind the Rover when they left the factory. I know my transmission is nothing like the earlier incarnations, so much better it is difficult to quantify without sounding like a salesman. HP Powermaxx Borg Warner 35 Max Stall Converter & Gearbox Package : Race Spec : 400+ HP